Method of producing stencil screens



United States Patent METHOD OF PRODUCING STENCIL SCREENS Oliver A.Short, Metuchen, N. J., assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing.Application May 24, 1956 Serial No. 586,923

3 Claims. (Cl. 101-1283) This invention relates to the production ofstencil screens, and more particularly it relates to a method ofproducing such screens that will resist elevated temperatures forextended periods of time.

Stencil screens for the application of inks, colors, ceramic pigments,and the like have been used for many years. Stencil screens generallyused in the ceramic industry are woven with fine metal, preferablystainless steel, wire to a mesh size of between about 150 to 175 meshesper linear inch. Such screens are prepared for printing by closing allof the interstices between wires except those which define the designwhich is intended to be printed. The design is printed on a surface byforcing semi-fluid or pasty printing material through the remaining openmeshes by means of an instrument commonly referred to as a squeegee.

These metal screens may be prepared for printing of simple designs byapplying cellulosic or similar films, from which the designs have beencut, onto the bottom surface of the screens. Where the screens are to bepre pared for printing more complicated designs or indicia, the screensare commonly coated with a uniform film of a photo-sensitive materialwhich is dried in complete darkness and then exposed to strong lightthrough a positive photographic film of the design or indicia. Afterexposure, the film protected by the design on the photograph remainssoluble and may be washed from the screen with a liquid, usually water,thus forming the open meshes of the screen. The remaining portions ofthe film are rendered insoluble by the actinic rays, thus forming theclosed screen meshes.

More recently, in the ceramic industry, ceramic colors are applied in aheated, molten state through stencil screens. In this process theceramic colors are dispersed in a wax or wax-resin vehicle which issolid at room temperature but semi-liquid or pasty at a temperature ofbetween about 120 to 122 C. The ceramic colors are molten and maintainedin the molten state on the stencil screen by application of heat. Theheat may be transmitted to the screen either by radiation, such as byinfra-red rays, by conduction or induction, such as by conducting orinducing an electric current through the stencil screen wires.

Such applications of heat to the stencil screens very materiallyshortens the screen life due to the poor heat resistance of the gelatinor polyvinyl alco-rol films, heretofore used for this purpose, on thescreens.

It is, therefore, an object of this invention to provide a new andimproved method of closing, or filling, the desired meshes of a stencilscreen with a heat-resistant organic film.

It is another object of this invention to close or fill the desiredmeshes of a stencil screen with a heat-resistant organic film by asimple method which will permit the formation of complicated design onthe screen.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by firstforming on a stencil screen a photographic negative of the desireddesign with filled meshes 2. of a soluble, light-hardened material,filling the open screen meshes of such design with a heat-resistantorganic film taken from the group consisting of polytetrafluoroethyleneand polysilicone resins, drying said heat-resistant organic film toinsolubilize the same, and dissolving from the screen said soluble,light-hardened material, thus leaving on the screen a photographicpositive of the desired design with filled meshes of a heat-resistant,insoluble polytetrafluoroethylene or polysilicone.

In forming on the stencil screen a photographic negative of the desireddesign with filled meshes of soluble light-hardened material, anymaterial may be used that in its light-hardened state after exposure tolight in the presence of a light-sensitizing agent will be soluble in aliquid which will not dissolve the subseqeuntly applied film ofpolytetrafluoroethylene or polysilicone. Substantially all commonly usedlight-sensitized, film-forming materials may be used for this purpose.Such material may be composed of polyvinyl alcohol, water-solublegelatin, agar-agar, polyglycuronic acid, albumin, watersoluble ethylcellulose, water-soluble polyvinyl acetals, and the like, sensitized tolight with conventional lightsensitizing materials such aslight-sensitive silver halides, light-sensitive bichromates, diazoniumcompounds, and the like.

The light-sensitized, film-forming material in aqueous solution isapplied as a uniform film on the surface of the stencil screen and driedin the darkroom. The dried film is then exposed to a strong lightthrough a negative photographic film or plate until the light passingthrough the negative has water-insolubilized the portions of the film soexposed. The remaining soluble portions of the film are then washed fromthe screen by spraying with cold or warm water as required. The screenis now.

provided with the desired design in photographic negative with open andfilled meshes.

The open meshes are then filled with an aqueous dispersion ofpolytetrafluoroethylene or with an ether solution of a heat-resistantpolysilicone which is then dried to set the dispersion or solution inthe form of a highly heat-resistant solid that is insoluble in mostcommon solvents.

The light-hardened, water-insoluble portions of the light-sensitizedfilm are then removed by treatment with a solvent or solution that willdissolve the same without dissolving the applied polytetrafluoroethyleneor polysilicone. In marginal portions of the screen design where thepolytetrafiuoroethylene or polysilicone overlies portions of thelight-sensitized film the former will easily be removed by a strongwater spray after the light-sensitized film is dissolved from thescreen.

The heat-resistant polysilicones useful in carrying out the process ofthis invention are the polysilicones free from carbon-to-carbon bonds asdisclosed in Rochow Patents Nos. 2,258,218, 2,258,219, 2,258,220, and2,352,974. Polytetratluoroethylene is a resin commonly known to behighly heat resistant up to 350 C. and is available in 40% to 60% solidsaqueous dispersion under the trade-name Teflon from I. do Font deNemours & Company, Wilmington, Delaware.

The heat-resistant polytetrafluoroethylenes useful in carrying out theprocesses of this invention are disclosed in the following patents:Berry 2,478,229, Sanders 2,520,173, and ()sdal 2,562,117 and 2,562,118.

The following examples are given to illustrate in detail specificmethods for the production of stencil screens of this invention. Thedetails set forth in these examples are not to be taken as limitative ofthe scope of this invention.

Example I A screen stencil of 10 inch by 20 inch dimension comprisingwoven, stainless-steel wire with 165 meshes per linear inch stretched onan aluminum frame is dipped into a solution of a light-sensitive coatingcomposition consisting of the following:

2002. polyvinyl alcohol, a 4% solution in water having a viscosity of20-25 centipoises; lgal. water; and 8 /2 oz. potassium bichromatesolution (saturated).

This dipping is carried out so that the coating composition lightlycontacts the screen surface and does not seep through the mesh. Thecoated screen is then dried, at a temperature of 8590 F, in completedarkness.

'Nextplace a photographic negative film of the desired design over theapplied film and direct a 35 ampere,

1500 candle power light toward said negative from a dis tance of 36inches for a period of about five minutes. After such exposure, thepolyvinyl alcohol film protected by the design of the photographicnegative will remain soluble while the remainder of the film has beenrendered water-insoluble by the actinic rays of the light. The screen isthen immersed in lukewarm water and then sprayed with water at roomtemperature, the spray being directed toward the film side of thescreen. The soluble polyvinyl alcohol is thus removed from the screen.

The screen is then dried and a 60% aqueous dispersion ofpolytetrafluoroethylene is applied by brushing onto the open meshportion of the screen. The screen is then dried at a temperature of 100C. and heated for ten minutes to a temperature of 200 C. to sinter theparticles of polytetrafluoroethylene and form a continuous film over thecovered meshes.

The light-insolubilized polyvinyl alcohol is then removed from thescreen by first soaking and then spraying the film side of the screenwith an aqueous hydrogen peroxide solution containing 35% H 0 In thisform the screen now has the open mesh in the desired positive pattern.The screen is then heated for ten minutes at a temperature of 375 C. toharden the polytetrafiuoroethylene. The resulting screen may be used atelevated temperatures for printing molten thermofluid ceramic colorsover long periods of time.

Example II 4% Patent No. 2,258,218. The screen is then heated and driedat a temperature of 200 C., after which the insolubilized gelatin isremoved therefrom by washing and spraying with a gelatin solvent.

The resulting screen may be used in a squeegee printing process at atemperature up to 250 C. for long periods of time without damage to thesilicone masking material.

Throughout the specification and claims, any reference to parts,proportions, and percentages refers to parts, proportions, andpercentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claims.

I claim:

1. The process of incorporating a design on a stencil screen whichcomprises forming thereon a photographic negative design with a solublelight-hardened material, filling the open screen meshes of such designwith a liquid organic film taken from the group consisting ofpolyvtetrafluoroethylene and polysilicone resins, drying said film toform a heat-resistant insoluble solid, and dissolving said solublelight-hardened material from said screen, thus leaving on the screen aphotographic positive of the desired design with filled meshes of saidheat-resistant material.

2. The process of incorporating a design on a stencil screen whichcomprises forming thereon a photographic negative design with a solublelight-hardened material, filling the open screen'meshes of such designwith a liquid film of polytetrafluoroethylene, drying said film to forma heat-resistant insoluble solid, and dissolving said solublelight-hardened material from said screen, thus leaving on the screen aphotographic positive of the desired design with filled meshes of saidheat-resistant material.

3. The process of incorporating a design on a stencil screen whichcomprises forming thereon a photographic negative design with a solublelight-hardened material, filling the open screen meshes of suchdesignwith a liquid film of a polysilicone resin, drying said film toform a heat-resistant insoluble solid, and dissolving said solublelight-hardened material from said screen, thus leaving on the screen aphotographic positive of the desired design with filled meshes of saidheat-resistant material.

References Cited in the file of this patent UNITED STATES PATENTS1,064,166 Pindikowsky June 10, 1913 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent'No, 2,860,576 November 18, 1958 OliverA., Short It is'hereby certified that error appears in the printedspecification of the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below,

Column 1, line 50, for ''122 C0" read 212 C0 line 57, for "applications"read application line 59, for "alcorol" read alcohol Signed and sealedthis 10th day of March 1959,

(SEAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Officer Comr'xissioner ofPatents

